The present disclosure relates to a developing device which supplies toner to an image carrier by using a toner carrier, and also relates to an electrophotographic image forming apparatus including such a developing device.
An electrophotographic image forming apparatus forms an electrostatic latent image by irradiating a circumferential surface of an image carrier (photosensitive drum) with light based on image information read from an original image or image information obtained, by transmission and so on, from an external device such as a computer. The image forming apparatus supplies toner from the developing device to the electrostatic latent image and forms a toner image, and then transfers the toner image onto a sheet. After the transfer processing, fixing processing is performed on the toner image, and then the sheet is ejected out of the image forming apparatus.
In recent years, more and more image forming apparatuses have come to be equipped with a color printing function and capable of high-speed processing. This progress has naturally caused image forming apparatuses to be configured increasingly more intricate, and further, higher-speed processing requires higher-speed rotation of a toner stirring member inside a developing device, and such high-speed rotation causes the developing device to tend to have positive internal pressure, which is higher than the ambient pressure. When toner is supplied to a photosensitive drum from a developing device having positive internal pressure, part of the toner leaks out of the developing device as scattered toner through an opening (a toner supply port) thereof which faces the photosensitive drum, and stains the interior of the image forming apparatus.
In particular, with a developing system that uses a two-component developer containing a magnetic carrier and a toner, and that also uses a magnetic roller (a developer carrier) for carrying the developer and a developing roller (a toner carrier) for carrying only the toner, toner that has not been used in the development is stripped off from the developing roller by a magnetic brush formed on the magnetic roller at a developing roller-magnetic roller opposing portion at which the developing roller and the magnetic roller face each other. This causes the toner to be likely to float in the vicinity of the developing roller-magnetic roller opposing portion, and the floating toner leaks out as scattered toner. Moreover, such floating toner accumulates in the interior of the developing device and forms lumps of toner, which fall down onto the developing roller, disrupting the thin layer of toner formed on the developing roller. This makes defects such as “dropping toner” more likely to occur, in which toner is not supplied to a portion where toner should be adhering on a circumferential surface of the photosensitive drum.
To overcome such defects as described above, some known developing devices prevent leakage of toner floating therein through an opening thereof by forcefully sucking in air into a duct disposed in an upper portion of a developing device through an air outlet port formed in an upper end portion of the developing device disposed to face a toner carrier